Pressure on Yahoo continued to grow after the internet search giant revealed that it was the victim of another huge hacking attack in 2013.

The company's shares closed down more than 6% on Wall Street on fears that a planned sale to Verizon was in doubt.

Politicians and regulators called for action after Yahoo disclosed on Wednesday that more than one billion user accounts may have been breached.

According to reports in the US, telecommunications giant Verizon, which agreed to buy Yahoo's core internet business in July for $4.8bn, is now trying to persuade Yahoo to amend the terms of the acquisition agreement to reflect the economic damage from the two hacks.